As NASA's Cassini dove close to Saturn in its final year, the spacecraft provided intricate detail on the workings of Saturn's complex rings, new analysis shows.

Although the mission ended in 2017, science continues to flow from
the data collected. A new paper published June 13 in Science describes
results from four Cassini instruments taking their closest-ever
observations of the main rings.

Findings include fine details of features sculpted by masses embedded
within the rings. Textures and patterns, from clumpy to strawlike, pop
out of the images, raising questions about the interactions that shaped
them. New maps reveal how colors, chemistry and temperature change
across the rings.

Like a planet under construction inside a disk of protoplanetary
material, tiny moons embedded in Saturn’s rings (named A through G, in
order of their discovery) interact with the particles around them. In
that way, the paper provides further evidence that the rings are a
window into the astrophysical disk processes that shape our solar
system.

The observations also deepen scientists’ understanding of the complex
Saturn system. Scientists conclude that at the outer edge of the main
rings, a series of similar impact-generated streaks in the F ring have
the same length and orientation, showing that they were likely caused by
a flock of impactors that all struck the ring at the same time. This
shows that the ring is shaped by streams of material that orbit Saturn
itself rather than, for instance, by cometary debris (moving around the
Sun) that happens to crash into the rings.

“These new details of how the moons are sculpting the rings in
various ways provide a window into solar system formation, where you
also have disks evolving under the influence of masses embedded within
them,” said lead author and Cassini scientist Matt Tiscareno of the SETI
Institute in Mountain View, California.

Enduring Mysteries

At the same time, new puzzles have arisen and old mysteries have
deepened with the latest research. The close-up ring images brought into
focus three distinct textures — clumpy, smooth and streaky — and made
it clear that these textures occur in belts with sharp boundaries. But
why? In many places the belts aren’t connected to any ring
characteristics that scientists have yet identified.

“This tells us the way the rings look is not just a function of how
much material there is,” Tiscareno said. “There has to be something
different about the characteristics of the particles, perhaps affecting
what happens when two ring particles collide and bounce off each other.
And we don’t yet know what it is.”

The data analyzed were gathered during the Ring Grazing Orbits
(December 2016 to April 2017) and the Grand Finale (April to September
2017), when Cassini flew just above Saturn’s cloud tops. As the
spacecraft was running out of fuel, the mission team deliberately
plunged it into the planet’s atmosphere in September 2017.

Cassini’s Visible and Infrared Mapping Spectrometer (VIMS) uncovered
another mystery. The spectrometer, which imaged the rings in visible and
near-infrared light, identified unusually weak water-ice bands in the
outermost part of the A ring. That was a surprise, because the area is
known to be highly reflective, which usually is a sign of
less-contaminated ice and thus stronger water ice bands.

The new spectral map also sheds light on the composition of the
rings. And while scientists already knew that water ice is the main
component, the spectral map ruled out detectable ammonia ice and methane
ice as ingredients. But it also doesn’t see organic compounds — a
surprise, given the organic material Cassini has discovered flowing from
the D ring into Saturn’s atmosphere.

“If organics were there in large amounts — at least in the main A, B
and C rings — we’d see them,” said Phil Nicholson, Cassini VIMS
scientist of Cornell University in Ithaca, New York. “I’m not convinced
yet that they are a major component of the main rings.”

The research signals the start of the next era of Cassini science,
said NASA’s Ames Research Center’s Jeff Cuzzi, who’s been studying
Saturn’s rings since the 1970s and is the interdisciplinary scientist
for rings on the Cassini mission.

“We see so much more, and closer up, and we’re getting new and more
interesting puzzles,” Cuzzi said. “We are just settling into the next
phase, which is building new, detailed models of ring evolution —
including the new revelation from Cassini data that the rings are much
younger than Saturn.”

The new observations give scientists an even more intimate view of
the rings than they had before, and each examination reveals new
complexities, said Cassini Project Scientist Linda Spilker, based at
NASA’s Jet Propulsion Laboratory in Pasadena, California.

“It’s like turning the power up one more notch on what we could see
in the rings. Everyone just got a clearer view of what’s going on,”
Spilker said. “Getting that extra resolution answered many questions,
but so many tantalizing ones remain.”

The Cassini-Huygens mission is a cooperative project of NASA, ESA
(European Space Agency) and the Italian Space Agency. JPL, a division of
Caltech in Pasadena, manages the mission for NASA’s Science Mission
Directorate in Washington. JPL designed, developed and assembled the
Cassini orbiter. The radio antenna was built by JPL and the Italian
Space Agency, working with team members from the U.S. and several
European countries.